Cell Compartmentation

Instructor Information

  • Name: Parkson Chong

  • Location: 617 MRB

  • Contact: pchong02@temple.edu

  • Phone: 2-4182

Objectives

  1. Compare and contrast prokaryotic and eukaryotic cells.

  2. Understand key organelles in a typical mammalian cell and their biochemical roles.

  3. Learn how biochemists fractionate cells to obtain pure organelle samples.

  4. Recognize the importance of metabolic compartments within cells.

  5. Comprehend the role of intracellular membranes in cell compartmentalization.

  6. Understand the varying metabolic functions of different cell types in the body.

Overview of Cell Structures

  • Biological molecules in living cells are organized into functional structures.

  • All cells possess membranes which are crucial for compartmentation.

Prokaryotic Cells

  • Characteristics:

    • Lack membrane-separated intracellular organelles.

  • Key Components:

    • Cell Membrane: The boundary of the cell.

    • Cell Wall: Provides shape and protection.

    • DNA: Located within the cell but not enclosed within a nucleus.

Eukaryotic Cells

  • Characteristics:

    • Contain numerous intracellular organelles separated by membranes.

  • Major Components:

    • Nucleus

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Mitochondria

    • Lysosomes

Membrane Structure of Eukaryotic Cells

  • A typical eukaryotic cell membrane consists of:

    • Peripheral and integral proteins

    • Cholesterol (adds fluidity)

    • Phospholipids and glycoproteins

  • Regions:

    • Hydrophilic region: Faces the aqueous environment.

    • Hydrophobic region: Located within the membrane bilayer.

Compartmentation in Eukaryotic Cells

  • Definition: A compartment is a distinct fraction of cellular volume physically and functionally separated by a membrane.

  • Mandatory Feature: Membrane presence is required to create a compartment.

  • Characteristics:

    • Defined by volume and surface area considerations.

    • Shape Variance: Can be spherical or irregular.

Intracellular Volume Distribution in Liver Cells (Hepatocytes)

  • Percent of Total Cell Volume:

    • Cytosol: 54\%

    • Mitochondria: 22\%

    • Rough ER: 9\%

    • Smooth ER + Golgi: 6\%

    • Nucleus: 6\%

    • Peroxisomes: 1\%

    • Lysosomes: 1\%

Membrane Surface Areas in Eukaryotic Cells

  • Distribution Trends:

    • Plasma Membrane: 2\% (Hepatocyte) to 5\% (Pancreatic Exocrine Cell).

    • Rough ER: 35\% (Hepatocyte) up to 60\% (Pancreatic Exocrine Cell).

    • Smooth ER: 16\% (Hepatocyte) vs. <1\% (Pancreas).

    • Mitochondrial Membranes: 7\% (outer) and 32\% (inner) in Hepatocytes.

    • Nuclear Membrane: 0.2\% (inner) and 0.7\% (outer).

    • Lysosome/Peroxisome membranes: 0.4\% each.

Obtaining Subcellular Organelles

  1. Homogenization:

    • Uses a tight-fitting pestle to disrupt tissue and release components like nuclei and mitochondria.

  2. Differential Centrifugation:

    • Step 1: Centrifuge at \sim 600 \times g for 10 minutes to pellet nuclei.

    • Step 2: Centrifuge supernatant at \sim 10,000 \times g for 20 minutes to pellet mitochondria, lysosomes, and peroxisomes.

    • Step 3: Centrifuge supernatant at \sim 100,000 \times g for 60 minutes to separate membranes (microsomes) from the soluble cytosol.

Detailed Organelle Functions

Nucleus

  • Marker: Presence of DNA.

  • Function: Site of chromosomes and DNA-directed RNA synthesis (transcription).

Endoplasmic Reticulum (ER)

  • Structure: Flattened tubular structures known as cisternae.

  • Rough ER: Studded with ribosomes; primary site of protein synthesis.

  • Smooth ER: Lacks ribosomes; primary site of lipid synthesis.

Mitochondria

  • Outer Membrane: Contains porins, permeable to molecules up to \sim 10\text{ kDa}.

  • Inner Membrane: Highly selective; contains Electron Transport Chain complexes and ATP synthase.

  • Cristae: Inward folds that increase surface area for ATP production. Higher cristae density indicates higher metabolic demand (e.g., heart muscle).

Lysosomes

  • Function: Degradation of intracellular and extracellular material via hydrolases.

  • Environment: Operates at an optimal low pH (\sim pH\ 5).

  • Outputs: Recycles proteins and lipids into amino acids, fatty acids, glycerol, and inorganic phosphate (P_i).

Golgi Complex

  • Functions: Protein and lipid glycosylation, sorting, and secretion.

  • Vesicle Transport Proteins:

    • COPI: Retrograde transport (Golgi to ER).

    • COPII: Anterograde transport (ER to Golgi).

    • Clathrin: Trafficking to lysosomes.

Cytoskeleton

  • Scaffolding: Provides shape and enables vesicle transport.

  • Filament Types:

    1. Microfilaments: Actin-based, \sim 7\text{ nm} diameter.

    2. Intermediate Filaments: Various proteins, \sim 10\text{ nm} diameter.

    3. Microtubules: Tubulin-based, \sim 25\text{ nm} diameter and \sim 50\text{ \mu m} long.

Metabolic Cooperation and Quantitation

  • Cooperation: Specific pathways (like glucose oxidation) require transport between the cytosol and mitochondria.

  • Molecular Scale: At a concentration of 1\text{mM}, there are roughly 100,000 molecules per mitochondrial particle. In a liver cell with 1,700 mitochondria, this equals 170,000,000 molecules per cell.

Quiz Questions

  1. Which cellular structures are bounded by a plasma membrane, include a nucleus and mitochondria?

  2. Identify the most highly curved membrane: Inner mitochondrial membrane.

  3. What is one major function of the Golgi apparatus? Protein sorting.